This variable will be set by bsd.port.mk to be the appropriate reference to the Motif library. Please patch the source of your port to reference this wherever the Motif library is referenced in the Makefile or Imakefile.
There are two common cases:
If the port refers to the Motif library as -lXm in its Makefile or Imakefile, simply substitute ${MOTIFLIB} for it.
If the port uses XmClientLibs in its Imakefile, change it to ${MOTIFLIB} ${XTOOLLIB} ${XLIB}.
Note that MOTIFLIB (usually) expands to -L/usr/X11R6/lib -lXm or /usr/X11R6/lib/libXm.a, so there is no need to add -L or -l in front.